This invention relates in general to processing of color video signals. In particular, this invention relates to method and apparatus for combining first and second color video signals into a single color video signal for recording or transmission in a reduced time period with minimal loss of signal information.
A standard color video signal such as the composite NTSC color television signal includes amplitude modulated luminance and quadrature related chrominance components which are frequency interleaved. The luminance component is transmitted at a full frequency bandwidth i.e. 4.2 Mhz., whereas the chrominance components are reduced in bandwidth. In the NTSC system approximately thirty frames of color information are transmitted each second with each frame comprising two interlaced fields. In order to record a composite television signal for home use, commercially available video cassette recorders (VCRs) utilize rotating magnetic heads to lay down oblique tracks on moving magnetic tape. A single video field is recorded on a single track length so that two tracks are required to record a complete video frame. If the frame is played back in a still frame mode, the playback heads will tend to bridge adjacent tracks of a frame causing annoying noise signals and jitter in the displayed picture. Moreover, with the trend towards more compact VCR's and the combination of the recorder and the camera into a lightweight, easy to handle, hand-held unit, the need exists to record the video signal on less and less recording tape. Thus, in U.S. Pat. No. Re. 29,999 entitled SYSTEM AND APPARATUS FOR RECORDING AND REPRODUCING TELEVISION VIDEO SIGNALS, reissued May 15, 1979, by A. Hirota there is disclosed a system for reducing by one-half the amount of magnetic tape required to record and play back a color video frame signal. As disclosed, only one of the two fields of each frame of a video signal is recorded on tape transported at one-half of the conventional speed. On playback, the field is played back twice in interlaced fashion to reproduce the video frame. This system results in a loss of vertical resolution since only half of the scanned lines of a video frame are reproduced.
It has also been proposed to transmit two video signals over a single cable channel by bandwidth compression techniques in a paper entitled "Cost Effective Bandwidth Compression Techniques for Increasing CATV System Channel Capacities" by R. L. Hess and H. C. Willard, of General Electric Co., Portsmouth, Va. As disclosed, the luminance components in adjacent scan lines are added to produce a signal which is transmitted at maximum bandwidth, and are also subtracted to produce another signal which is transmitted at reduced bandwidth. The sum and difference luminance signals are modulated in quadrature on an RF carrier. The two chrominance components are bandwidth limited and averaged for the two lines and modulated in quadrature on a color subcarrier frequency. The luminance and chrominance quadrature signals are sent within the time period of a single line so that the time period of a second line may be used to transmit a second different video signal. Although this system reduces the transmission time of a video frame signal, recording and playback may produce cross-modulation between the quadrature luminance signals resulting in crawling patterns in the edges of objects, and objectionable disturbances in picture reproduction. The quadrature chrominance signal may be subjected to time base errors in recording and playback which cause color bands in the picture.
Thus, there exists a need for video signal processing techniques which will permit the recording or transmission of a greater amount of video information on a given amount of record material or in a reduced transmission channel without loss of picture resolution and color fidelity and without the production of picture disturbances and image degradation. There is also a need in consumer VCRs to provide the ability to play back a single video frame in a still mode without image disturbances and degradation.